1. Field of the Invention
The present invention relates to a thermal transfer type color printer, and more particularly to a thermal transfer type color printer which prints out a color image of high quality without color deviation.
2. Prior Art
Thermal transfer type color printers are well known, and are conventionally used to print out the color image which is displayed on a display screen of a personal computer or a display screen for computer aided design (CAD) or computer aided manufacturing (CAM), system. This thermal transfer type color printer performs color printing to printing paper color inks of plural colors painted on an ink film. This thermal transfer type color printer has relatively simple construction, so that it is advantageous in saving much time and labor in maintenance and management thereof. Hereinafter, description will be given with respect to an example of this thermal transfer type color printer by use of FIGS. 1 to 3.
In FIG. 1, 1 designates a printer unit. At rear face of this printer unit 1, a printing paper supply roll 3 on which a printing paper 2 is wound up, is supported by a bracket 1a so that the roll 3 can freely revolve. As shown in FIG. 2, a plenty of feed holes 2a, . . . are disposed in a longitudinal direction at both edge portions of the printing paper 2. As shown in FIG. 1, the printing paper 2 which is pulled out from the supply roll 3 is partially wound and guided by a guide roller 4 and a platen roller 5, and then the printing paper 2 is pulled out from the printer unit 1. The above-mentioned guide roller 4 and platen roller 5 are provided for conveying the printing paper 2. Sprocket pins 4a, . . . are formed at outer peripheral face of the guide roller 4, while other sprocket pins 5a, . . . are formed at outer peripheral face of the platen roller 5. The diameters of those sprocket pins 4a and 5a are smaller than those of the feed holes 2a of the printing paper 2. In addition, those sprocket pins are provided in connection with the feed holes as shown in FIG. 2. The shaft edge of platen roller 5 is interconnected with a motor (not shown), so that the platen roller 5 can revolve around the shaft line thereof in a forward direction (i.e., Y1 direction) and in a negative direction (i.e., Y2 direction). This revolution of the platen roller 5 is transmitted to the roller 4 via a belt (not shown). This belt is wound by the guide roller 4 and platen roller 5 so that the rotational speed of the guide roller 4 will be equal to that of the platen roller 5 but the rotational direction of the guide roller 4 will be inverse to that of the platen roller 5.
Meanwhile, under the above guide roller 4, an ink film supply roll 7 on which an ink film 6 is wound up is mounted so that this supply roll 7 can freely revolve. As shown in FIG. 3, the color inks of yellow, magenta and cyan colors are sequentially painted in turn on the ink film 6, wherein the yellow, magenta and cyan colors are the primary colors of color image printing. The ink film 6 which is pulled out from the supply roll 7 is partially wound around outer portion of the printing paper 2 which is partially wound by the platen roller 5. Further, the ink film 6 is wound up by an ink film winding roll (hereinafter, simply referred to as "winding roll") 9a of ink roll (hereinafter film transporting means 9. This ink film feeding means 9 is provided for transporting the ink film 6 in its longitudinal direction, wherein this means 9 comprises the winding roll 9a and a drive unit (not shown) which revolves the winding roll 9a in Y3 direction.
Under the above-mentioned platen roller 5, a thermal head 10 is provided. This thermal head 10 is provided so that it can be pressed against and also separated from the outer peripheral face of the platen roller 5. In the case where this thermal head is pressed against the ink film 6 wound around the outer peripheral face of the platen roller 5 while the printing paper 2 is inserted between this ink, film and the platen roller, the thermal head gives thermal pulses of thermal energy to the ink film 6 so that the color ink painted on the ink film 6 is transferred onto the printing paper 2.
In the thermal transfer type color printer having the above-mentioned constitution, color printing is performed accordance with the following procedure.
First, the platen roller 5 is revolved in the forward direction (i.e., Y1 direction) so that the printing start position of the printing paper 2 is transported to the printing position of the thermal head 10, while the winding roll 9a of the ink film transporting means 9 is revolved in the Y3 direction so that the head position of the yellow color ink painted on the ink film 6 (see FIG. 3) will be coincided with the printing position of the thermal head 10. Next, the thermal head 10 is pressed against the ink film 6 which is wound around the outer peripheral face of the platen roller 5 via the printing paper 2. Then, the platen roller 5 is revolved forward, and the printing paper 2 is transported toward the printing paper discharging side in the state where the printing paper 2 is piled with the ink film 6. In this state, the thermal pulses are adequately given to the thermal head 10, so that the yellow color ink is transferred on the printing paper 2.
After completing the yellow color printing as described above, the thermal head 10 is separated from the ink film 6. Next, the platen roller 5 is revolved in the forward direction so that the printing paper 2 will be separated from the ink film 6. Then, the platen roller 5 is revolved in the reverse direction (i.e., the Y2 direction) by the predetermined distance so that the printing paper 2 is transported toward the printing paper supplying side (i.e., the right side in FIG. 1), whereby the printing start position of the printing paper 2 is returned back to the printing position of the thermal head 10. At the same time, the winding roll 9a of the ink film transporting means 9 is slightly revolved in the Y3 direction so that the head position of magenta color ink painted on the ink film 6 (see FIG. 3) will be coincided with the printing position of the thermal head 10.
Similar to the case of yellow color printing, the thermal head 10 is pressed against the ink film 6 and the platen roller 5 is revolved in the forward direction so that the magenta color ink is transferred on the printing paper 2. Thus, the magenta color printing is completed. Next, similar to the above-mentioned printing procedure, the printing paper 2 is transported in the reverse direction and the ink film 6 is transported. Then, the cyan color ink is transferred on the printing paper 2. As a result, the color image is printed on the printing paper 2.
Meanwhile, in the case where the thermal head 10 transfers the ink of ink film 6 on the printing paper 2 in the conventional thermal transfer type color printer, the thermal contraction must be effected on the ink film 6 due to the heat generated by the thermal head 10, so that the ink film must be wrinkled. For this reason, the ink film 6 must be folded so that certain portion of the ink film will be adhered to the printing paper 2. When the thermal transfer is performed in such state, it is difficult to transfer the ink of the folded portion of the ink film 6 to the printing paper 2. Hence, the transferred image will have the color deviation, or there will be the un-transferred portion of the image. So, there is a disadvantage in that the desirable image can not be obtained. This must be remarkably emerged as the color deviation of image in the color printing for transferring plural colors, which deteriorates the image quality.
On the other hand, since the three color inks painted on the ink film 6 are sequentially transferred on the printing paper in the conventional thermal transfer type color printer, the platen roller 5 is reversely revolved to the predetermined position so that the printing position of the thermal head 10 will be repeatedly coincided with the printing start position. This deteriorates the accurate coincidence between the printing position of the thermal head 10 and the printing start position of the printing paper 2 at the printing start timing of each color, and this also lowers the precision for repeatedly transferring each color ink. Therefore, there is a problem in that the color image having high quality can not be obtained.
More specifically, the feed holes 2a, . . . of the printing paper 2 are matched with the sprocket pins 5a, . . . of the platen roller 5, and the platen roller 5 is revolved forward and backward so that the printing paper 2 can move forward and backward along the outer peripheral face of the platen roller 5. When the platen roller 5 is revolved forward so that the thermal head 10 is pressed against the ink film 6 in the printing period, the frictional force in the Y2 direction as shown in FIG. 3 is applied between the printing paper 2 and the outer peripheral face of platen roller 5 or the ink film 6 due to the pressure applied by the thermal head 10. Thus, the printing paper 2 is transported in the Y4 direction shown in FIG. 2, wherein each of the sprocket pins 5a, . . . is regularly matched with each of the feed holes 2a, . . . thereof.
On the contrary, in the case where the printing paper 2 is transported in the direction reverse to the Y4 direction (i.e., in the backward direction), the thermal head 10 is separated from the ink film 6 and the pressure thereof is released, so that the above frictional force will not be applied. For this reason, the printing paper 2 is transported in the state where each of the sprocket pins 5a, . . . is irregularly matched with each of the feed holes 2a, . . . thereof. Even when the platen roller 5 is revolved backward to the predetermined position, the printing paper 2 must be transported in the unstably deviated manner in the range of the difference between the inner diameter of feed hole 2a and the outer diameter of sprocket pin 5a. Therefore, it becomes impossible to coincide the predetermined printing position of the thermal head 10 with the printing start position.
In order to prevent the wrinkles of ink film from being formed and also prevent the color dislocation from occurring, the conventional printer (as disclosed in Japanese Patent Application No. 57-180335) provides tension applying means by which tension force is applied to the ink film in the printing. As for this tension applying means, electromagnetic brake which is driven to be on or off is used. In addition, in order to adjust braking force with slip control of such electromagnetic brake, the voltage applied to the brake is controlled to become lower than the rated voltage. However, due to the mechanical variation of the brake and voltage variation of power source, it is difficult to accurately adjust the tension force applied to the ink film.